Acid is a status effect which inflicts acid damage on the affected character until the duration of the effects wears or if the character dies from its effects. The amount of damage and duration depends on the means by which it is inflicted. This status effect can be inflicted by the spell Melf's Acid Arrow.
The means to prevent any character from this effect are acid and magic resistance and the spell Spell Immunity. The items which can prevent this are Potion of Magic Blocking and the Scroll of Protection from Magic.